Method for the production of plastic skins by powder sintering and corresponding sintering tool

ABSTRACT

The invention relates to a method of production for plastic skins by pulverizing, whereby the powder is placed on a forming tool and a plastic is formed by sintering; a partial area of the forming tool, at least during a first pulverizing step, is made, inaccessible for the powder by means of a mask with a preferably inflatable sealing edge, the forming tool being provided with a separating web along one edge of the partial area whereon the mask rests during the first pulverization step with the sealing edge. The invention also relates to a corresponding sintering tool and a plastic part which comprises a plastic skin, which is produced according to said method of production, on one surface thereof. The inventive method makes it possible to produce two-coloured plastic skins, which are usually used as surface decorations, while effectively avoiding scrap production.

The invention relates to a method for the production of plastic skins bypowder sintering in accordance with the preamble of the main claim andto a corresponding sintering tool in accordance with the preamble ofclaim 8, as well as to a plastic part which has on one surface a plasticskin produced using this production method.

Production methods of this type serve to produce generally soft plasticskins which can be used as surface decorations for moulded plasticparts, such as automotive vehicle interior trims for example, especiallyinstrument panels. In order to form such a moulded plastic part, suchplastic skins can be back-sprayed or back-foamed and possibly in thisprocess be connected to a support as an additional component. Inconventional powder sintering methods, a forming tool which has a shapewhich corresponds to a desired shape for the plastic skin or for acorresponding moulded plastic part is brought by heating to atemperature which lies above the melting point of the plastics materialused, and placed on a powder box which contains the plastics material inthe form of powder. Then the powder box and the forming tool can be sorotated that the powder falls into the forming tool, where it forms theplastic skin by sintering, possibly after the described steps have beenrepeated a number of times. Non-melted powder residues can in each casefall back into the powder box after a further rotation of the powder boxand forming tool.

In more recent applications of plastic skins as surface decorations,surface properties which differ in areas are desired, especially thedemarcation of partial areas by a different colour. In order to achievethis with a powder sintering method of the described type, there is aknown way of making one partial area of the forming tool inaccessiblefor the powder during a first pulverisation step, such that a firstplastics material layer is produced which leaves out this partial area,and then following this with a second pulverisation step using adifferent plastics material, in which the partial area is alsopulverised and a second plastics material layer is produced which hasthe desired surface properties different from those of the firstplastics material layer. Leaving out the partial layer during the firstpulverisation step is achieved in corresponding production methodsaccording to the prior art in that a possibly inflatable seal is pressedalong a separating line delimiting the partial area, onto the formingtool or onto a separating web arranged along the separating line on theforming tool. For this purpose, these seals are arranged on an edge ofan additional wall of the powder box, by which means the surface of theforming tool which receives the plastic skin being produced issubdivided into different areas. For the first pulverisation step, thenat least one of the areas is kept free of powder; for the secondpulverisation step generally a different powder box is used which thendoes not have to have any such seal.

The described production method entails various disadvantages. Itrequires a powder box which is designed in an extremely expensive wayand has at least one additional wall and a seal placed on it for whichvery narrow geometric tolerances must be observed, in order to achieveeffective covering of the partial area of the forming tool. Even in ahigh-precision tool, sealing defects can practically not be excluded,for example due to a forming tool being not placed quite exactly ontothe powder box, which can lead to powder displacements in the firstpulverisation step. Even with separating lines of simple geometry, aproduction of scrap is thus practically unavoidable. A production methodaccording to the prior art becomes very problematic as soon as aseparating line between surface areas having different properties isintended to follow a three-dimensional contour. Even slight curvaturessuch as can be present for example on the scoops of instrument panels,lead to a scrap production of roughly 20%.

The object underlying the invention, therefore, is to develop aproduction method of this type and a corresponding sintering tool, bymeans of which the mentioned disadvantages can be avoided. Inparticular, a reliable separation of a partial area of the forming toolin as simple a manner as possible, preventing powder displacement,should be possible for a first pulverisation step even in the case ofmore complicated surface geometries of the forming tool andthree-dimensional courses of an edge of the partial area.

This object is accomplished according to the invention by a productionmethod and a sintering tool according to the characterising features ofclaims 1 or 8 in conjunction with the features of the preambles ofclaims 1 or 8. Advantageous embodiments of the invention arise with thefeatures of the subordinate claims.

Due to the separating web extending along a generally enclosed edge ofthe partial area, it becomes possible to cover the partial area for thefirst pulverisation step with a mask which is solely secured to theforming tool. The mask, which does not necessarily have to followexactly each surface contour of the forming tool, is for this purposeclamped to the forming tool with its sealing edge, preferably aninflatable bead, abutting against the boundary web, such that said maskis held by the separating web. Thus an extremely effective sealing ofthe partial area is achieved with a very simple sintering tool. Inparticular, a simple powder box without additional features can be used.Without great outlay and with greatly reduced effects oftolerance-related deviations by comparison with the prior art, due tothe effective sealing powder displacement into the partial area isreliably avoided during the first pulverisation step, even if the edgeof the partial area follows a three-dimensional contour. If the firstpulverisation step is followed, after removal of the mask, by a furtherpulverisation step using a different plastics material, differing forexample in colour or mechanical properties, plastic skins can beproduced, effectively avoiding scrap, which in a partial area havecorrespondingly differing properties, and in comparison with the priorart, more general courses of separating lines between areas of differentcolour or any other properties are possible. Here a single productionstep, described as a pulverisation step, can include the application ofpowder to the heated forming tool, melting-on and removal of excesspowder, also in a multiple sequence, for example by repeated rotation ofpowder box and forming tool.

In comparison with the production of two-coloured plastic parts or thosewhich have areas differing in some other surface properties and whichare formed from a plurality of parts put together, the use of a plasticskin produced using the described production method and having differingsurface properties in at least one partial area, has the advantage thata one-part embodiment is possible, thus eliminating a cause of undesiredrattling or squeaking. In comparison with other possible ways ofproducing plastic parts with two-coloured soft surfaces, such as bypartial painting of the surface or by welding two single-colouredplastic skin parts, the invention has the advantage of a considerablyreduced outlay.

Apart from the production of two-coloured plastic skins or plastic skinswhich have some other differing properties in certain areas, thedescribed method is also advantageous for producing plastic skins withrecesses. Compared with subsequent cutting-out or stamping-out of apartial area to be recessed, with the production method described herequite considerable amounts of material can be saved in such a case.

What is proposed therefore is an advantageous method for the productionof plastic skins by powder sintering, in which powder is applied to aforming tool, where it forms a plastic skin by sintering, a partial areaof the forming tool being made inaccessible for the powder at leastduring a first pulverisation step by a sealing device, and the sealingdevice embodied as a mask being laid for the first pulverisation stepwith a preferably inflatable sealing edge against a separating webextending along an edge of the partial area, the mask being thus securedsolely to the forming tool in typical embodiments of the invention. Theapplication of the powder to the forming tool can take place by rotatingthe forming tool—typically together with a powder box—exploiting gravityand/or a centrifugal force.

Even more secure attachment of the mask to the forming tool can beachieved in the described production method if the separating web isundercut so that it forms through an overhang to the partial area agroove open in that direction. For attaching the mask to the formingtool, the sealing edge can then be introduced into this groove so thatin the groove it abuts against the separating web and the mask isprevented from sliding out by this form-fit connection.

Particularly suitable as material for the mask is silicon, the elasticproperties of which facilitate securing the mask to the forming tool,and which permits very good sealing of the partial area particularly ifthe mask has an inflatable sealing edge. For inflating the sealing edge,the mask can have a preferably separable supply line for compressed airor the like.

The forming tool, which due to a corresponding three-dimensionalcontouring of a surface receiving the produced plastic skin, permits theproduction of a plastic skin which is suitably pre-shaped for later useas surface decoration for a moulded part, with possibly a separatingline which, depending on the selected course of the separating web, alsofollows a three-dimensional contour, between surface areas of differingproperties, can have a shell for receiving the plastic skin beingproduced, which shell predetermines the contour and has a wall thicknessof between 2 mm and 6 mm. With a wall thickness of the shell of between2 mm and 4 mm, a particularly good compromise is achieved betweensufficiently high stability and simultaneously not too high heatingcapacity, whereby heating and cooling for pulverisation, sintering andsubsequent curing is made possible with low outlay.

The forming tool or the shell described in the previous paragraph can beproduced from nickel, a material which is particularly suitable for thispurpose because it permits easy detachment of the finished plastic skinfrom the forming tool, avoiding damage. Advantageous furthermore, inaddition to the good thermal conductivity of nickel, is also thepossibility of particularly simple production of an appropriate formingtool. Such a nickel shell can be produced on a deposition mould forexample by electrodeposition or by nickel vapour deposition. Theseparating web on the forming tool can also be very simply realised, forexample by introducing a finished nickel web profile into the depositionmould in such a way that nickel being deposited grows on the webprofile, the web profile forming the separating web of the finishedsintering tool. Another possibility is welding the nickel separating webonto a web stump which has been ground flat. Other metals which havesimilar properties, such as copper for example, can also be consideredfor the forming tool, in other production methods for the forming toolalso as alloys.

An expedient further development of the described sintering toolprovides for the forming tool to be double-walled, i.e. to have, forexample, two similarly shaped shells at a small spacing from each other.A cavity thus produced between the two shells can serve to guide aheating medium, by which means the necessary heating of the forming toolfor sintering can be very practically achieved, for which purposeexpensive additional method steps would otherwise be required, such asfor example the introduction of the forming tool into a furnace. Oil isfor example suitable as a heating medium. For cooling the forming toolafter pulverisation in order to cure the plastic skin, cold oil or someother preferably fluid and corrosion-avoiding substance can serve in thesame way as the coolant.

The forming tool is expediently combined with a powder box onto whichthe forming tool can be placed as tightly as possible. To simplify thedescribed production method, the sintering tool can be mounted so as tobe rotatable about a horizontal axis, for example by arranging theforming tool in a rotating frame.

If the surface of the forming tool which receives the plastic skin as itis being produced is designed with differing graining inside and outsidethe partial area, with the described production method plastic skins canbe produced which differ in sharply defined areas not only in theircolour or other material properties, such as haptic properties forexample, but which also have differing surface graining in these areas.An effect dividing the surface of a plastic part into different areascan thus be further intensified. Naturally the case is also possiblethat the forming tool is grained only inside or only outside the partialarea; then plastic skins which are grained in some areas and smooth inother areas are obtained.

In order to guarantee secure attachment of the mask and thus goodcoverage of the partial area for the first pulverisation step, theseparating web is preferably to be designed with a height of between 2mm and 7 mm. Too high a separating web should be avoided so that no toogreat material excess is produced at the separating line of the finishedplastic skin. Particularly good results are achieved with a height ofthe separating web of between 3 mm and 5 mm. The groove formed by anundercut separating web can have a depth of between 0.2 mm and 2 mm,preferably between 0.3 mm and 1 mm, for secure clamping of the mask, sothat in a further pulverisation step without the mask, for producing agap-free plastic skin, the groove is also reached by the powder. For assharp as possible a separating line between the various areas of theplastic skin being produced, the separating web should not be too wide.A separating web which is still sufficiently stable can have a width ofbetween 1 mm and 6 mm, preferably between 2 mm and 4 mm.

The sealing edge of the mask is advantageously to be of dimensionscorresponding to the separating web. In the inflated state, the sealingedge can have for example a diameter of between 5 mm and 20 mm,preferably between 10 mm and 15 mm. In other areas, on the other hand,the mask can also have a lower thickness, however care should be takento ensure that powder falling onto the mask does not melt there, whichcan be prevented by a mask which is in turn not too thin. To avoidsintering of the powder falling onto the mask, arranging a thermalshield on the mask is also possible; with this a thickness of the maskof up to 20 mm is expedient.

For forming a plastic part, for example an instrument panel or someother interior trim part for automotive vehicles, with a plastic skin,produced using the described production method, as surface decoration,the plastic skin can be back-foamed. Thermoplastic polyurethane isparticularly suitable as the back-foaming compound. During theback-foaming, the plastic skin can be connected to a support, usually aplastics material support, which is foamed round or on in the sameworking step, this support providing the plastic part with increasedstability and bearing capacity. A foam layer thus produced behind theplastic skin preferably has a thickness of between 3 mm and 10 mm; thusplastic parts can be realised which are neither too heavy nor toosensitive.

The invention is explained with the aid of embodiments depicted in FIGS.1 and 2. These show:

FIG. 1 two successive method steps of a production method according tothe invention, a sintering tool and a plastic skin being produced beingshown in cross-section, and

FIG. 2 also in cross-section, a sintering tool for producing a plasticskin for trimming an instrument panel.

In FIG. 1 can be recognised a forming tool 1, which has an undercutseparating web 2, the separating web 2 forming a closed boundary of apartial area on the forming tool 1. The separating web 2 has a height 3of roughly 4 mm, a width 4 of roughly 3 mm and, through an overhangtowards the partial area, forms a groove 5 having a depth 6 ofapproximately 0.5 mm. The forming tool 1 itself, of which only a detailcan be seen in the figure, consists of nickel and has a wall thickness 7of approximately 3 mm. The forming tool 1 forms overall a shell-likeopen shape with a surface contour which corresponds to a desired contourfor a plastic skin to be produced. Important for this normally is asurface contour predetermined by a moulded plastic part for which theplastic skin is intended to serve as surface decoration. The separatingweb 2 can also have a course which follows a three-dimensional contour,in the depicted cross-sections substantially perpendicular to the planeof the drawing.

For the method step depicted at the top of FIG. 1, a silicon mask 8having an inflatable sealing edge 9 is so secured to the forming tool 1that the mask 8 covers the partial area which is surrounded by theseparating web 2. The mask 8 is here solely secured to the forming tool1, and for this purpose clamped with the sealing edge 9 in the groove 5.By inflating the sealing edge 9, which thus attains a diameter ofroughly 10 mm, secure attachment of the mask 8 to the forming tool 1 isobtained as well as a good seal between the sealing edge 9 and thegroove 5 or the separating web 2 respectively. The mask 8, which has ashape corresponding to the partial area, has a thickness of roughly 3 mmapart from the thicker sealing edge.

Through the production method, two steps of which are illustrated inFIG. 1, a plastic skin can be produced which has in a partial area adifferent colour from that outside the partial area or which alsodiffers in respect of other properties such as for example hapticproperties inside the partial area. For this purpose, the mask 8 issecured to the forming tool 1 for a first pulverisation step, as isdepicted at the top of FIG. 1. After heating the forming tool 1 to atemperature of roughly 250° C., for example by blowing through hot airat a temperature of roughly 400° C., the forming tool 1 is placed withthe mask 8 onto a powder box which is not shown in the figure. Thepowder box contains plastics material for the first pulverisation stepin powder form, the melting point of this plastics material lying belowthe temperature of the heated forming tool 1. By rotating the powderbox, with the forming tool 1 placed on it, by roughly 180° about ahorizontal axis, around which the sintering tool is rotatably mountedfor this purpose, the powder is made to fall onto the forming tool 1,where it forms by sintering a first plastics material layer 10 as aresult of the high temperature of the forming tool 1. The partial areacovered by the mask 8 remains free of plastics material during thisprocess. After a further rotation by roughly 180°, excess powder fallsback into the powder box. Repeating the described steps possibly once ora number of times gives the first plastics material layer 10 a desiredthickness. The method step depicted at the top of FIG. 1 shows theforming tool 1, the mask 8 and the first plastics material layer 10after the steps described so far.

Then the mask 8 is removed, to which end the sealing edge 9 can beemptied. A second pulverisation step follows which resembles thejust-described first pulverisation step, but in which some otherplastics material is used which differs, for example in colour, from theplastics material used for the first pulverisation step. As the mask 8is now absent, the initially covered partial area on the forming tool 1is also reached by the powder. For the second pulverisation step, adifferent powder box can expediently be used which is similar to thepowder box used first but contains powder of a different plasticsmaterial. During the second pulverisation step, a second plasticsmaterial layer 11 is produced which becomes connected to the firstplastics material layer 10 outside the partial area, but inside thepartial area forms a single layer. The method step shown at the bottomof FIG. 1 shows the forming tool 1, the first plastics material layer 10and the second plastics material layer 11 at this point in time. Thefirst plastics material layer 10 and the second plastics material layer11 together form a plastic skin which, preferably after cooling of theforming tool 1, can be removed from the forming tool 1. Due to thecovering of the partial area in a first pulverisation step, the finishedplastic skin has a two-coloured surface structure or a surface whichdiffers in some other properties in at least one partial area. If theforming tool 1 has a differently grained surface inside or outside thepartial area, a further contrast between different areas on the plasticskin can be realised by differing graining.

A method related to the production method described with the aid of FIG.1 is obtained if the second pulverisation step is omitted. A plasticskin is then obtained which has a corresponding recess in the partialarea. Corresponding plastic skins can be used for example as decorationsfor interior trim parts which are intended to leave an area free. Byusing the production method hereby described, subsequent cutting-out orstamping-out of the recess becomes superfluous in this case and in thisway unnecessary material losses can be avoided with at least aconsiderable reduction of an otherwise unavoidably large volume ofstamping waste.

To produce a moulded plastic part having a plastic skin, produced asdescribed, as the surface decoration, the plastic skin can for examplebe back-foamed with polyurethane, expediently with simultaneous foaminground of a plastics material support. Due to the described productionmethod, the finished moulded plastic part can then have a surface whichis, for example, two-coloured with a separating line between areas ofdiffering colour which, if this is desired, can also without any problemhave a complicated geometry and especially a geometry following athree-dimensional contour.

In FIG. 2 is shown in cross-section a complete forming tool 1 having amask 2 placed on it for a first pulverisation step. The forming tool 1and the mask 2 are constituent parts of a sintering tool for producing aplastic skin which is intended to serve as surface decoration for thetrim for an instrument panel. The plastic skin obtains through a methodof the above-described type a different colour in an area covered by themask from that outside this area. In this figure can also be seen aninflatable sealing edge 9 of the mask 2, which is produced from siliconas in the previously described case. As the figure shows, the mask, whenit is secured to the forming tool l, does not necessarily have to followevery contour of the forming tool 1. The mask 2 is again secured to theforming tool 1 by being clamped into a groove 5, which is not visiblehere and which is formed by an undercut separating web 2, also notrecognisable here. The forming tool 1, a nickel mould produced by anelectroforming method in a galvanic shell and having a wall thickness ofroughly 3 mm, has a shape which corresponds approximately to a finishedtrim for an instrument panel. The depicted cross-section wouldcorrespond in the finished automotive vehicle to a plane standingperpendicular to the direction of travel in the region of a scoop lyingon the left in the figure and serving to cover instruments.

1. Method for the production of plastic skins by powder sintering, in which powder is applied to a forming tool where it forms a plastic skin by sintering, a partial area of the forming tool being made inaccessible for the powder by means of a sealing device, at least in a first pulverisation step, characterised in that the forming tool has a separating web along an edge of the partial area and in that a mask having an inflatable sealing edge serves as a sealing device, the mask abutting with the sealing edge against the separating web during the first pulverisation step and being secured solely to the forming tool.
 2. Production method according to claim 1, characterised in that the first pulverisation step is followed by at least one additional pulverisation step, preferably to produce a plastics material layer which is of a different colour from a first plastics material layer produced in the first pulverisation step, the mask being removed for the additional pulverisation step.
 3. Production method according to claim 1, characterised in that the separating web is undercut and thus forms a groove which is open towards the partial area and in which the sealing edge comes to rest during the first pulverisation step.
 4. Production method according to claim 1, characterised in that the forming tool consists of nickel, at least at a surface which receives the plastic skin being produced.
 5. Production method according to claim 1, characterised in that the sealing edge of the mask consists of silicon or a duroplastic elastomer.
 6. Production method according to claim 1, characterised in that the plastic skin is provided with areas of differing graining due to the different graining of a surface receiving the plastic skin being produced, inside and outside the partial area.
 7. Production method according to claim 1, characterised in that, due to a three-dimensional contour of a surface of the forming tool receiving the plastic skin during the powder sintering process, said plastic skin obtains a correspondingly three-dimensional contour, and possibly due to a three-dimensional course of the separating web, on the plastic skin a separating line having a correspondingly three-dimensional course is produced between surface areas of differing colour and/or graining.
 8. Sintering tool for producing plastic skins by powder sintering, which has a forming tool with a surface for receiving a plastic skin being produced and which has a sealing device for separating a partial area of the surface, characterised in that the forming tool has a separating web on the surface along an edge of the partial area, and in that the sealing device is designed as a mask having a preferably inflatable sealing edge, which is to be secured to the surface in such a way that the partial area is covered by the mask and the sealing edge abuts against the separating web.
 9. Sintering tool according to claim 8, characterised in that, when the mask is secured to the surface, it is solely secured to the forming tool.
 10. Sintering tool according to claim 8, characterised in that the separating web is undercut and forms a groove which is open towards the partial area and in which the mask, when secured, abuts with the sealing edge against the separating web.
 11. Sintering tool according to claim 8, characterised in that the forming tool has a shell with a wall thickness of between 2 mm and 6 mm, preferably between 2 mm and 4 mm, for receiving the plastic skin being produced.
 12. Sintering tool according to claim 8, characterised in that the forming tool is double-walled for guiding a preferably liquid heating medium and/or coolant in a cavity between two walls.
 13. Sintering tool according to claim 8, characterised in that it has at least one powder box on which the forming tool may be placed, the sintering tool being preferably mounted so as to be rotatable about a horizontal axis.
 14. Sintering tool according to claim 8, characterised in that the forming tool consists of nickel, at least at the surface.
 15. Sintering tool according to claim 8, characterised in that the sealing edge of the mask consists of silicon or a duroplastic elastomer.
 16. Sintering tool according to claim 8, characterised in that the mask has a thickness of between 1 mm and 6 mm, preferably between 2 mm and 4 mm and/or the sealing edge, when inflated, has a thickness of between 5 mm and 20 mm, preferably between 10 mm and 15 mm.
 17. Sintering tool according to claim 8, characterised in that the separating web has a height of between 2 mm and 7 mm, preferably between 3 mm and 5 mm and/or a width of between 1 mm and 6 mm, preferably between 2 mm and 4 mm.
 18. Sintering tool according to claim 10, characterised in that the groove has a depth of between 0.2 mm and 2 mm, preferably between 0.3 mm and 1 mm.
 19. Sintering tool according to claim 8, characterised in that the surface has differing graining inside the partial area.
 20. Sintering tool according to claim 8, characterised in that the surface has a three-dimensional contour.
 21. Sintering tool according to claim 8, characterised in that the separating web has a three dimensional course.
 22. Plastic part which has on one surface a plastic skin produced by means of a production method according to claim 1, the plastic skin preferably having inside at least one area a surface of a different colour and/or different graining and at least one plastics material layer less than outside this area.
 23. Plastic part according to claim 22, characterised in that the plastic skin is back-foamed, preferably with polyurethane, the plastic part having in a particularly preferred manner a support which is foamed round or on.
 24. Plastic part according to claim 22, characterised in that the plastic skin is back-sprayed, preferably with a back-sprayed compound containing polypropylene. 